Keywords: 
• organophosphate pesticides /
• isobologram /
• concentration additive isobole /
• combined effect /

Abstract: The study attempts to explore the application of a new combined effects model of binary mixtures of organophosphate pesticides using FH interpolation method to isobologram. In this study Caenorhabditis elegans (C. elegans) selected as the representative species were exposed to dichlorvos (DDVP) and monophosphate (MCP) to measure the respective single toxicity and their combined toxicity. Based on the results of 4 hours' acute toxicological test of single pesticide on C. elegans, the principles of isobologram were applied to plot the concentration additive isoboles of 30%, 50% and 70% mortality effects on C. elegans and to determine the confidence interval (CI); moreover, the experimental results of the combined effects of binary mixtures were adopted to establish the combined effects model of the binary mixtures of organophosphate pesticides by using FH interpolation method; finally the contour lines and the combined effects surface of the mixtures were taken to plot the observed isoboles of 30%, 50% and 70% mortality effects on C. elegans. The results showed that the 4 h-LC₃₀ (lethal concentration), 4 h-LC₅₀, 4 h-LC₇₀ and CI of DDVP and MCP were 35.40 (26.85-41.64), 43.55 (36.00-50.76), 53.58 (45.99-64.95) μmol·L^-1 and 13.15 (9.12-16.07), 16.96 (13.35-20.55), 21.87 (18.11-28.36) mmol·L^-1 respectively. The observed isoboles of 30%, 50% and 70% mortality effects on C. elegans were within the CI of the concentration additive isoboles, which indicated that the combined effects of DDVP and MCP were additive demonstrating the agreement with the result obtained via multivariate statistical analysis. This study is significant in the prediction of observed isobole of any effect under the combined effects of binary mixtures of organophosphate pesticides and thus provides a new method for comprehensive and effective evaluation of the combined effects of binary mixtures.